During the manufacture of paper, a dried web of paper coming from a dry end section of a papermaking apparatus is initially wound on a reel spool to form a parent roll which typically is temporarily stored for further processing. Subsequently, the parent roll is unwound and the web of paper is converted into a final product form.
In winding the paper web into a large parent roll, it is vital that the roll be wound in a manner which prevents major defects in the roll and which permits efficient conversion of the roll into the final product, whether it be boxes of facial tissue sheets, rolls of bath tissue, rolls of embossed paper towels, and the like. Ideally, the parent roll has an essentially cylindrical form, with a smooth cylindrical major surface and two smooth, flat, and parallel end surfaces. The cylindrical major surface and the end surfaces should be free of ripples, bumps, waviness, eccentricity, wrinkles, etc., or, in other words, the roll should be "dimensionally correct." Likewise, the form of the roll must be stable, so that it does not depart from its cylindrical shape during storage or routine handling, or, in other words, the roll should be "dimensionally stable." Defects can force entire rolls to be scrapped if they are rendered unsuitable for high speed conversion.
Many defects can be introduced by improper winding of the paper web onto the parent roll, especially when winding high bulk, easily-compressible, soft tissue webs. A large number of such defects are discussed and shown in photographs in an article by W. J. Gilmore, "Report on Roll Defect Terminology - TAPPI CA1228," Proc. 1973 Finishing Conference, Tappi, Atlanta, Ga., 1973, pp. 5-19. Inadequate web stress near the core of the roll may cause the outer regions of the roll to compress the roll inwardly, leading to buckling in a starred pattern, commonly called "starring", as described by James K. Good, "The Science of Winding Rolls", Products of Papermaking, Trans. of the Tenth Fundamental Research Symposium at Oxford, September 1993, Ed. C. F. Baker, Vol. 2, Pira International, Leatherhead, England, 1993, pp. 855-881. Furthermore, starring causes the release of the tension of the web around the core that normally provides sufficient friction between the core and adjacent layers of the web. This loss of friction can result in core "slipping" or "telescoping", where most of the roll (except for a few layers around the core and a few layers around the outermost regions) moves en masse to one side with respect to the axis of the roll, rendering the roll unusable.
Current commercially available hard nip drum reels of the type with center-assisted drives, as described by T. Svanqvist, "Designing a Reel for Soft Tissue", 1991 Tissue Making Seminar, Karlstad, Sweden, have been successfully used to wind rolls of compressible tissue webs having bulks of up to about 8 to 10 cubic centimeters per gram, while avoiding the above-mentioned winding problems, by reducing the nip force and relying mainly on the in-going web tension control through modulation of the center-assisted drive for the coreshaft. However when using such methods to wind tissue sheets having bulk of 9 cubic centimeters per gram or higher and a high level of softness, as characterized, for example, by an MD Max Slope of about 10 kilograms or less per 3 inches of sample width, these problems will recur. These winding problems are accentuated when attempting to wind large rolls with diameters from about 70 inches to about 150 inches or greater, particularly at high speeds.
Without wishing to be bound by theory, it is believed that when a web is brought into a nip formed between the parent roll and a pressure roll, two major factors besides the in-going web tension affect the final stresses inside a wound roll. Firstly, the portion of the parent roll in the nip is deformed to a radius which is smaller than the undeformed radius of the parent roll. The expansion of the parent roll from its deformed radius to its undeformed radius stretches the web and results in a substantial internal tension increase from the set tension of the web going into the nip.
Another factor is sometimes called the "secondary winding" effect. A portion of the web is added to a roll after it passes first through the nip between the parent roll and the pressure roll. It then passes under the nip repeatedly at each rotation of the parent roll while more layers are added on the outer diameter. As each point near the surface of the roll reenters the nip, the web is compressed under the nip pressure, causing air in the void volume of the web to be expelled between the layers. This can reduce the friction between the layers sufficiently to allow the layers to slide tighter around the inner layers, as described by Erickkson et al., Deformations in Paper Rolls, pp. 55-61 and Lemke, et al., Factors involved in Winding Large Diameter Newsprint Rolls on a Two-Drum Winder, pp 79-87 Proc. of the First International Conference on Winding Technology, 1987.
The tension in each layer as it is added to the parent roll causes a compression force exerted by the outer layer to the layers underneath, and thus the cumulative effect of compression from the outer layers will normally cause the web at the region around the core to have the highest interlayer pressure. The secondary winding further adds to this pressure. Soft tissue is known to yield when subjected to compression, thus absorbing some of the increases in pressure to the extent that it loses its ability to deform. Consequently, the cumulative pressure can rise at a steep rate to excessive levels that can cause a wide variation in the sheet properties unwound from the parent rolls.
Unfortunately, the internal pressure and web tension gradient that exists along the radius of a conventionally wound parent roll, while successful in preventing dimensional stability problems, can lead to undesired variability in the properties of the web. High tension in some regions causes some of the machine direction stretch to be pulled out during winding, and high internal pressure results in loss of bulk. Upon unwinding, regions that have been stretched more by high tension in and after the nip will have lower basis weight because of longitudinal stretching of the web. These changes in crucial web properties lead to variability in product quality and difficulties in converting operations.
Compensating for the internal pressure build-up, according to the above-mentioned method described by T. Svanqvist, can be carried only to a certain extent. As the density and strength of the web material is reduced much lower than the levels cited, uncertainties in the magnitude of frictional forces in the winding apparatus and other factors which change during the course of winding a roll make precise nip loading control very difficult. Alternatively, loss of control of the winding process can result in a reversal in tension gradient that can lead to the starring and core slippage problems described above.
In conventional nip winding, the reel spool is pressed into engagement with the reel drum by a pair of hydraulic actuators. Strain gage type sensors are mounted on the hydraulic actuators to sense the amount of strain in the actuators, which is then used to determine the nip load between the reel drum and growing paper roll. Although such an arrangement may be preferable because of the attendant advantages of nip winding (i.e., obtaining a sufficiently high tension in the wound paper), it has been difficult to accurately maintain and control the nip load (which is very important for the reasons presented above). The limits of conventional strain gage sensors and uncertainties in the frictional forces of the apparatus (such as, for example, variations in the sliding friction of the hydraulic actuators or associated carriages for moving the reel spool) have imposed limits on the accuracy of the nip loading, which in turn places limits on the quality and size of the parent rolls and types of paper which can be wound. Efforts to address these problems have been made for improving the accuracy of nip load control during a change-over procedure, as described in published PCT Application WO 97/22543 by Olsson. Olsson attempts to improve nip load control during a change-over, when a new reel spool is moved into position and the paper begins to be wound onto the new spool, by locating force-sensing devices on the primary and secondary arms in an attempt to directly measure the nip load during the change-over. However, Olsson does not address the problem of accurately controlling nip load during a winding operation, in which, particularly for soft paper grades such as tissue, the indentation of the drum into the roll for a given nip load is constantly changing as the thickness of the paper on the roll builds. A nip load control scheme that may be useful during a change-over procedure may not be optimum for a winding operation.
Accordingly, there is a need in the industry for winding apparatus which can be used for various grades of paper, including soft and delicate grades of paper like tissue. Such an apparatus should afford the advantages of nip winding but also provide accurate and effective nip loading so that the quality and size of the parent rolls can be improved.